Monarch Population Status

Tagging will tell

As you may recall, I posted three reports on the development of this years’ monarch population from 1March to 10 June to the Monarch Watch Blog (see below). I stopped reporting in early June due to the lack of meaningful monarch observations that can be used to track how the population is developing. There is weather data to work with, but the temperatures seemed ok so I ignored the rainfall which had already been excessive. That was a mistake. Rainfall, if it persists for days – and it did over wide areas – can make a difference by limiting the number of hours and days during which females can lay eggs. In other words, it can reduce realized fecundity because there are only so many days in a female’s life and there is no way to fully recover from time lost. This idea is covered in a paper by Zalucki and Rochester (2004). The overall effect of delayed and reduced oviposition in June would be a reduction in the number of second-generation adults that emerge in July, and that, in turn, could result in a reduction of the number of offspring that become third and fourth generation migrants in August and September.

Most of the monarchs in each migration originate from breeding areas north of 40N latitude – imagine a line from St Joseph, Missouri to Philadelphia, Pennsylvania. We have learned from recovered tags that timing of departure and geographic origin of each migrant largely determines whether it will reach the overwintering sites in Mexico. It’s important to be early, or at least on time, relative to the timing of migrations under average weather conditions. This migration could be late. The “tagging will tell”, but to assess lateness, we will need taggers to extend their tagging efforts as late as possible into the migration.

As most of you know, the size of the last generation is a function of the number of eggs laid from about 20 July to 5-10 August as well as the quality of the milkweeds and the weather. The adults from that oviposition typically emerge throughout August into early September. This year the emergence could be delayed due to a cold front that moved into the northern breeding area starting on the 4th of August. Overnight temperatures dipped into the 50s in many areas limiting the number of hours for larval development. It’s possible that development has been pushed back by at least 10 days. This means that monarchs that would normally be on the wing in early to mid-August are still larvae or pupae at this writing (19 August). Hopefully, that’s the case, and we are simply dealing with a late emergence and migration.

This has happened before. The temperatures in August 2004 in Minnesota and the Upper Midwest were the lowest in the 30year record. The migration was extraordinarily late and the number reaching Mexico was lower than expected. Again, tagging will tell if that outcome is duplicated during this migration.

Oh, and there is one more thing. Significantly higher-than-average September temperatures have occurred in 17 of the last 30 years and in 13 of those years, the population has decreased from that of the previous year. These high temperatures also delay migrations. Elevated September temperatures have occurred in 6 of the last 8 years and are becoming the new normal. Should such conditions occur again this September, those could also reduce the number of monarchs arriving at the overwintering sites.

References
Taylor, O. R. 2024. Monarch population development in 2024: Part 1.
monarchwatch.org/blog/2024/04/02/monarch-population-development-in-2024-part-1

Taylor, O. R. 2024. Monarch population development in 2024: Part 2.
monarchwatch.org/blog/2024/05/23/monarch-population-development-in-2024-part-2

Taylor, O. R. 2024. Monarch population development in 2024: Part 3.
monarchwatch.org/blog/2024/06/28/monarch-population-development-in-2024-part-3

Zalucki, M.P. & Rochester, W.A. (2004) Spatial and temporal population dynamics of Monarchs down-under: lessons for North America. The Monarch Butterfly: Biology and Conservation (ed. by K. Oberhauser and M. Solensky), pp. 219–228. Cornell University Press, Ithaca, New York.